Everything to know about speedometers

A speedometer is an essential part of your dash—it’s how you know how fast you’re travelling. Without it, you can’t know whether you’re going the speed limit, or if you’re potentially going over. They’re easy to take for granted, but it makes sense to know how they work, and if/when you need to have yours checked.

Let’s take a look at how they function.

Why is a speedometer important?

Speeding is the number one cause of accidents on Australian roads, so knowing your speed is essential for your safety.

In fact, speeding just 5 km above the limit in an urban area doubles the risk of a crash, so having your speedometer properly calibrated is critical to accurately gauging your speed.

There are a few moving parts involved with the function of a speedometer, as well as aspects of your car that can alter how accurate it is; but it does need to be checked from time to time.

How does a speedometer work?

Your car’s speedometer measures how fast its wheels are spinning, and uses this information to display on the dashboard how far it will travel in an hour. Depending on the type of speedometer, this can be a digital display or a dial with numbers.

The two main types of speedometer are mechanical or electronic, which refers to how they work, not what you see on the dash.

Mechanical speedometer:

In mechanical speedometers, the pointer on the dial is connected to a metal ‘speed cup’ which sits around a magnet.This magnet is attached to a cable that runs directly to your car’s driveshaft.

When the driveshaft turns, so does the cable. As the cable spins, the spinning magnet creates an electromagnetic current in the air between the magnet and the cup called an ‘eddy current’. The cup is effectively ‘dragged’ around by the current, despite having no physical connection with it.

So the pointer doesn’t spin around wildly, the cup connects to a spring that is tensioned against it. As the cup spins, it pushes on the spring, bringing the pointer up as the speed increases.

Electronic speedometer:

Electronic speedometers are slightly less complex as they use small magnets that are attached to the car’s driveshaft next to magnetic sensors. As the magnets spin past the sensors with the driveshaft, they emit an electric pulse that the sensors count. The sensors detect the speed of the pulses, and convert this into a speed reading on the dashboard.

Do car speedometers show actual speed?

Speedometers are calibrated to show either the exact speed, or a slightly higher speed than you’re travelling as a safety precaution.

Australian regulations state the speedometer cannot show a speed less than the actual speed of the vehicle. However, the displayed speed can be 10% over the actual speed, plus 4 km/hr. This means you may have a bit of wiggle room, but it’s safer to err on the side of caution.

What about GPS speedometers?

GPS speedometers can be more accurate than mechanical speedometers in certain circumstances, as they measure speed more directly.

While mechanical speedometers rely on the tyres being the correct pressure, the temperature being within a certain range, the car not being overloaded etc, GPS measures the actual distance travelled, divided by the time taken.

That said, GPS speedometers do need a clear view between your car and the sky, and sometimes can lose signal. They aren’t great with twists and turns, and do better on straight stretches of road. While they can be accurate, it’s best to stick with your dash speedometer—as a GPS display can lag.

If you’re looking for speedometer accuracy in Australia, you’re better off properly calibrating the one in your car.

Signs your speedometer isn’t accurate

Often the ‘check engine’ light will come on when there’s faulty speedometer issues. This could come on for many other reasons as well, but it’s good to look out for in this case.

What if the dial isn’t moving at all? This could be due to worn out connecting cables or speed sensor issues. It could even be your engine control unit malfunctioning.

Brand new tyres, pumping them up or upgrading your wheel size can also impact the accuracy of your speedometer.

Ways to do a speedometer check

So how do you know your speedometer is right? There are a few methods:

Where can I get my speedometer checked?

Whether it’s an issue with a cable, a blown fuse, or you’ve changed the size of your tyres, your mycar mechanic can calibrate and check your speedometer for you.

No matter where you are in Australia, mycar has diagnostic services (including a speedometer check) to keep your car in good nick. We have a vast network of workshops all over the country, so help is never far away.

Find a store near you or book your speedometer check online today!